Means and method of cold bending sheet metal



Aug. 18, 1953 A. L. SCHOELLERMAN 2,649,128

MEANS AND METHOD OF COLD BENDING SHEET METAL Filed Feb. 24, 1948 2 Sheets-Sheet 1 I N V EN TOR. Azw/v L. Suwanee/mu 1953 A. 1.. SCHOELLERMAN 2,649,128

' MEANS AND METHOD OF GOLD BENDING SHEET METAL Filed Feb. 24, 1948 2 Sheets-Sheet 2 3m LowEn Lml'r FOR lcoZ FoRMma usma FLAT M0659 Panel-l.

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Patented Aug. 18, 1953 PAT OFFICE MEANS AND METHOD OF GOLD BENDINGL SHEET METAIJ 2. Glaimsi. (CL. 153-481) My invention relates to: a means andmethod' of: bending sheet metal and, more particularly,. to a means and method of reducing minimum; bend radii when. thin. sheets of aluminum alloy, such asi'l5-Stal,.are.-bent in the cold.-state. Theiinven tion isrparticularly adaptablefor useL-in cold bends aluminum alloy flanges-on apower; brake, for usein airframe fabrication.

75 Stal. alum-inumalloycomes in sheet or extruded stools and is,. at present, the highest. strength: aluminum alloy used in the industry. Inthe: designation 75-Stal', 753 indicates the chemical analysis or alloy which: in the. present example is: Copper, l.2-2.0%; iron, .'7%; silicon, .5%; manganese, .3%='; magnesium, 2.1-2.9%; zinc, 51-61%; chromium, GAB-0.4%; titanium, 0.2%; other elements totaling 0.15%; the remainder of the whole-beingaluminum The- "t designates that the stock has been fully heat treated, and the al ('A'lclad indicates that the stock has asurface coating'of commercially pure aluminum or analloy thereof. In strength, 75 Stal tests as: follows? Tensile strength, 823000 p; s; i.; yield strength, 72,000 p i. shear strength, 4bZOOO -493000 p.v s. i; Brinell hardness factor 05 1 50 (measured with-10mm. ball-la F5115- ther, 75 Stal: has anendurance limitof 24,000

Under normal circumstances, relatively large radii are: required toform 75 Stal aluminum alloysheet for example, by cold bending; This quiresth'ei use: of wide flanges to obtain edge distance'. for rivets, and strength is lost'due to large radii. H'eretof'ore;v toobtain smaller radii recourse mustbe had tothe use of annealed stock which is veryexpensive, or to the use of sheets heated; to b'etween 300-400 duringf'ormi'ng. This latter method represents an extreme workihg hazard and the ideal temperatures are difiicult to maintain accurately;

Accordingly; it is an object of the-present invention to provide a means and method of reducing' existin minimum bend radii in unannealed alloy sheet stock when cold forming is practiced;

In the aircraft industry; large: numbers of alloy sheet flanges are formed on a power brake utilizing a spaced bottom support for the sheet and. then applying pressure from above with a round nose punch, the pressure being applied approximately midway between thelower sheet. supports. The punch is. machined with the curvature desired in the final bend, which is, in most cases, 90. In cold forming, 75 Stal, for example, there is a minimum bend radius for sheets of various thicknesses where the bend can be safely made; Cold. bends on smaller radii cause cracking: at the bend and cannotbe made with conventional round nose punches.

In accordancewith the present. invention; it was found that actually the bend: made in. the sheet did not conform to the radiusof the: round nosepunch but was smaller, and itisanotherrobject of the present invention to provide: a means" and method of modifying'the. nose contour. ofa

coldi bending punch to the end that minimum bend radii in sheet stock can be reduced from those heretofore obtained.

The invention can be more fully understood by reference to the drawings in which:

Figures 1-3 inclusive are diagrams showing the action ofa round nose punch on sheet stock duringthe formation of abend in the sheet.

Figured is a diagram showing'a preferred type of punch used to obtain smaller bend radii.

Figure 5 is a diagram showing the permissible variation in the punch contour.

Figures 6 -8 inclusive are diagramsshowing the action of a" punch modified in accordance with the present invention.

Figure 9 1's 2: coordinatediagram showlngminlmum bend radii for different die radii and different sheet thicknesses;

Figure 10 is a diagram showing the invention as used on arrofiset punch.

Referring'to Figures- 1-3 inclusive, showing the actionof a conventional round nosed punch on sheet stock when cold formed in apower brake; alower die I is placed on the brake bed having spaced uprights 2' terminatingin rounded sheet supporting'surfaces 3'. Sheet stock 4 is placed on the uprights 2* and a conventional round nosed punch 5 is mounted on the brake to be moved downwardly'under powerbetween uprights 2, thus bending the sheet. The lower die I and punch 5 are generally longer than the-bend length desired to bemade in thesheet. The radius in the final bend desired is used for developing the rounded surface of the punch.

InzEigure; 1. the conventional. round nose punch is lowered vertically to start the application of pressure on the sheet-4- at a point the' sheet bends slightly. In Figure 2 the punch is lowered fin-trier to form the sheet at the punch radius and the pressure area widens on both sides" of point Asthefull depth of punch penetration is approached; however, I have fbun'dj that the sheet material leaves the die below point A,.. the sheet bearing on the punch only on. each side. of

point" A as shown in Figure 3. Thus, maximum bending stresses are set up at point A and the actual bend made has a smaller radius than the punch. It will be noted in Figure 3 that for a 90 bend, a lesser angle is made to allow for springback to 90. However, this spring-back does not restore the proper radius at A nor does it afiect the over-stressing of the sheet at A. These stresses cause cracking and even actual separation of the sheet below the minimum bend radius, as has been determined by experience with sheets of different thicknesses and compositions. In

other words, almost the entire bending stress is concentrated at or near point A and this causes failure at that point at definite minimum radii.

I have found, however, that by suitably modifying the punch contour that the bending stresses can be distributed over a wider area, thus permit-- ting bends of smaller radii to be cold formed. In addition, this same modification will cause the sheet to more closely assume the radius machined on the punch.

A punch modified in accordance with the present invention is shown in Figure 4 where the end of the punch 5 has lateral surfaces 5 rounded to a desired radius R and then has a fiat T of extent X formed on the end thereof normal to the direction of force application. Extensive experimentation has proven that, for minimum radii, there is a correlation between radius R and fiat extent X that can be expressed as a preferred relationship for a wide range of sheet thicknesses, as follows While the above is the preferred relationship, I have found that some variation is allowable as shown in Figure 5. The range can be up to i-.015X. The space between lines X+ and X showing the useful range of X. I

The action of the punch as modified in accordance with the present invention is quite different from that of a round nosed punch, and is shown diagrammatically in Figures 6-8 inclusive, comparable as to general punch position with Figures 1-3 inclusive, respectively.

In Figure 6, the punch, as modified by flat 1 has just started to apply pressure to the sheet 4 and the pressure is applied substantially along I the fiat. As shown in Figure '7, as the punch moves further downwardly, the sheet material leaves the flat very early in the bending operation creating two pressure areas at areas B and C. The material is then forced to bend at these latter points along the lateral radii of the punch nose, as shown in Figure 8. In this manner, substantially a true radius is formed around the bend and the bending stresses are uniformly distributed.

The reduction in minimum bend radii that can be obtained by cold bending with the punch embodying the present invention shows a striking reduction over those obtained by a conventional round nose punch and is shown in Figure 9 for '75 Stal alloy sheet for various thicknesses. It will be seen that by the use of the flat end nose punch of the present invention, the allowable minimum bend radii curve E is significantly smaller than design allowables for round nose punches curve F. The minimum radii for 100% forming with the punch of the present invention is shown in curve G.

The punch of the present invention can also be applied to offset punches as shown in Figure 10.

Here the flat is machined on the portion of the onset punch first making contact with the sheet.

While I have described my invention as being accomplished by machining a fiat on the advancing end of the punch I wish it to be distinctly understood that the use of a leading area on the punch having a radius greater than the radius of the remainder of the punch is satisfactory to perform the invention and is deemed the full equivalent of the flat (infinite radius) within the scope of the appended claims. For example, the invention has been successfully practiced when the advancing larger radius is 1 /8 or more times the radius of the remainder of the punch. Obviously, however, the flat is easier to machine and is therefore the preferred form.

What is claimed is:

1. In a power brake for cold bending a metal sheet of '75 Stal, and other materials having substantially the same bending, tearing, and rupturing characteristics as '75 Stal, into a curve of the maximum order of and having a true radius of the order of 2.5 to 4 times the thickness of the sheet, the combination comprising: a punch having an end area initially engageable with a sheet to be bent and side areas extending laterally from said end area and subsequently engageable with the sheet, said side areas being curved about a common center to a true radius equal to the desired true radius of the bend to be effected and said end area having a radius ranging from 1%; times said true radius to infinity and a chordal width of times said true radius plus or minus 1.5%; a pair of sheet supports providing parallel fulcrums spaced a greater distance apart than the major width of said punch between said side areas plus twice the thickness of the sheet; and means for forcing said punch against a sheet bridged across said supports to bend the sheet around said side areas by contact of the sheet solely with said fulcrums and said punch.

2. The method of cold bending a metal sheet of 75 Stal, and other materials having substantially the same bending, tearing, and rupturing characteristics as 75 Stal, into a curve of the maximum order of 90 and having a true radius of the order of 2.5 to 4 times the thickness of the sheet, the step comprising: supporting the sheet on parallel fulcrums spaced apart a distance greater than twice the desired true radius of the bend to be efiected plus twice the thickness of the sheet and forcing a punch against the sheet between said fulcrums to bend the sheet by contact solely with said fulcrums and said punch, said punch having an end area initially engageable with the sheet and side areas extending laterally from said end area and subsequently engageable with the sheet, said side areas being curved about a common center to said true radius and said end area having a radius ranging from 1% times said true radius to infinity and a chordal width of times said true radius plus or minus 1.5%.

ALVIN L. SCHOELLERMAN.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Dingledine Aug. 7, 1888 Phillips Mar. 12, 1889 Schoen Feb. 7, 1893 Armstrong Oct. 9, 1894 Bischofi June 26, 1900 Forslund Feb. 19, 1918 Langford Sept. 2, 1919 Spencer July 20, 1926 Number Number Name Date Leis Mar. 22, 1932 Salzman Feb. 25, 1936 Leis Apr. 13, 1937 Robertson Jan. 4, 1949 FOREIGN PATENTS Country Date Great Britain of 1900 Germany Jan. 2, 1923 Germany Feb. 6, 1925 

